Add support for automatic field tagging.

[mbrancato]

Having every struct support a redis tag is both tedius and requires explicit tailoring for go-redis. By default, the reflect package adds a json tag. Or, in the case of encoding/xml, they just create a fallback field name when there is no tag.

https://github.com/golang/go/blob/813aad51d947b207f5bed4511c479c0921870b3d/src/encoding/xml/typeinfo.go#L193

It would be nice if I could hand HSet() an arbitrary struct, and it would go ahead and choose the field names like some other packages do. Currently, go-redis skips untagged fields as if they were tagged as "-".

go-redis/commands.go

Lines 105 to 108 in c0ab781

	tag := typ.Field(i).Tag.Get("redis")
	if tag == "" || tag == "-" {
	continue
	}